1. Field of the Invention
The present invention relates to a battery charger and, more particularly, to a battery charger for providing both constant current and constant voltage outputs suitable for charging batteries requiring constant current charging, such as nickel-cadmium (NiCd) and nickel metal hydride (NiMH), as well as batteries requiring both constant current and constant voltage charging, such as lithium ion batteries. The battery charger includes a pulse width modulator (PWM) for controlling a power transistor to provide a constant current or a constant voltage output to the battery being charged as a function of the battery characteristics. The battery charger may include dual pockets for charging two modular batteries at a time. In the dual pocket application, charging is divided between the two pockets on a time slice basis. In such a configuration, the power dissipation of the power supply and the power transistors used for supplying charging current to the pockets is monitored and controlled.
2. Description of the Prior Art
Various portable devices and appliances, such as cellular phones, require rechargeable batteries. Various types of rechargeable batteries are known to be used in such applications. For example, nickel-cadmium (NiCd), nickel metal hydride (NiMH), as well as lithium ion batteries are known to be used. Because of the different charging characteristics of such batteries, different battery chargers are required. For example, both nickel-cadmium (NiCd), as well as nickel metal hydride (NiMH), require constant current charging. On the other hand, lithium ion batteries require constant current charging up to a certain voltage value and constant voltage charging thereafter. Because of the different charging requirements, different charging circuits are often required.
Standard battery packs normally consist of one or more battery cells disposed in a modular housing with external contacts for easy and convenient coupling with the portable device in which it is used. Smart battery packs, in addition to the battery cells, normally include a memory storage device which contains information regarding the characteristics of the battery as well as the battery type. Some smart battery packs are known to include a microcontroller which allows communication by way of a bi-directional communication line with the battery charger regarding various battery characteristics. Examples of such smart battery packs are disclosed in: "Smart Battery Specifications", .sup..COPYRGT. Duracell Inc., Intel Corporation, hereby incorporated by reference. Because of the differences between the standard battery packs and the smart battery packs, different chargers are used for the smart battery packs and the standard battery packs.
Battery chargers for charging batteries which require constant current charging and batteries which require constant current and constant voltage charging, such as lithium batteries, are known in the art. Battery chargers are also known that are adapted to automatically sense the type of battery connected to the battery charger and provide the appropriate charging characteristic. As mentioned above, such battery chargers are used for various portable devices, such as cellular phones. Cellular phone battery chargers are commonly available as single pocket and dual pocket devices. Dual pocket devices are known to be used for charging a spare battery, as well as the battery connected to the cellular phone. Unfortunately, with known dual pocket battery chargers, each pocket is known to be treated independently. In particular, in situations in which batteries to be charged are disposed in both pockets, the battery in the active pocket is normally fully charged before any servicing of the battery in the other pocket is done. Lithium ion batteries are known to take 3-4 hours to charge. Should a second battery be placed in the inactive pocket while a lithium battery is being charged in an active pocket, the second battery could remain in the inactive pocket for 3-4 hours before charging is even commenced. If the second battery also happens to be a lithium battery, it could take from 6-8 hours for the second battery to be charged from the time the second battery is inserted in the inactive pocket. Unfortunately, the end user will normally not be aware of such a limitation in the charging system.